Conventionally, application of fuel cells using a fuel gas containing hydrogen and an oxidizing gas to automobile and so on has been studied, because the fuel cells are environmentally friendly and clean power generators which generate only water by power generation. In addition, application of the fuel cells to a cogeneration system which can achieve a high-energy utilization energy by recovering exhaust heat generated in power generation, by storing the heat in a heat accumulator, and by supplying the heat to a hot water load and the like as well as by supplying the electric power generated in the power generation to a specified power load has been studied.
By the way, when the operation of the fuel cell system is shut down, the fuel cell keeps a predetermined temperature (e.g., about 70 degrees C.) for some time after the fuel cell shut-downs power generation. A fuel cell system is known, which is aimed at efficiently making use of heat kept in the fuel cell after the power generation terminates (see Patent document 1).
In the fuel cell system disclosed in Patent document 1, after supply of a fuel gas to a fuel cell and supply of an oxidizing gas to the fuel cell are stopped when the power generation of the fuel cell is shut down, a heat medium circulating means (pump) is operated to recover heat of the fuel cell with a heat medium if a temperature of the heat medium (cooling water) outflowing from the fuel cell, which is detected by a temperature detector, is higher than a predetermined threshold (e.g., about 60 degrees C.), and the operation of the heat medium circulating means is stopped if the temperature of the heat medium is lower than the predetermined threshold.